As a process for producing batteries, a method wherein a positive electrode and a negative electrode are laminated with a separator for preventing a short-circuiting between the electrodes being sandwiched therebetween or a positive (negative) electrode, a separator, a negative (positive) electrode, and a separator are laminated in this order; an electrode/separator laminated body is obtained by winding; the electrode/separator laminated body is inserted into a cell; an electrolysis solution is injected into the cell; and the cell is sealed has heretofore been known (see Patent Publications 1 and 2, for example).
In using the thus-obtained battery, there is a problem that, when the battery is left to stand in an abnormally high temperature environment or overcharged or when a short-circuiting occurs between the electrodes inside or outside the battery, the battery is abnormally heated to cause the electrolysis solution inside the battery to spout out of the battery, sometimes resulting in fracture.
Meanwhile, particularly in a production of laminated batteries, a method of adhering electrodes and a separator to each other by using a polyvinylidene fluoride resin solution as an adhesive agent and removing a solvent used for the resin solution under a reduced pressure is employed in many cases. However, this method has problems such as complicated process steps, difficulty in regulating quality of products, and insufficient adhesion between the electrodes and the separator (see Patent Publication 3, for example).
Also, a porous film to be used for separators of batteries is produced, for example, by a method of drawing a molded sheet at a high draw ratio (see patent Publication 4, for example). Therefore, the battery separator made of such porous film has problems that the battery is remarkably shrunk in the high temperature environment where the battery is abnormally heated due to an internal short-circuiting or the like, or, in some cases, the separator itself is melted or broken to fail to function as the separator between the electrodes.
In order to improve the battery safety, it is important to achieve both of heat resistance and a reduction in heat shrinkage ratio of the battery separator in the high temperature environment. In view of such object, in order to suppress the heat shrinkage ratio of the battery separator in the high temperature environment, a method of producing a porous film by a method that does not include the drawing process in the production step has been proposed (see Patent Publication 5, for example). With this method, however, it is difficult to achieve sufficient strength since the porous film is not drawn. As described above, the conventional methods do not realize the simultaneous achievement of the improvement in heat resistance and the reduction in heat shrinkage ratio for the purpose of preventing the melting and broken of the separator in the high temperature environment.
Also, a lithium rechargeable battery that uses a conventional polyethylene porous film as a separator has difficulty in maintaining its initial performance in the case where the lithium rechargeable battery is used in a laptop personal computer (hereinafter referred to as laptop PC), used in a high temperature environment such as in a car in summer, or left to stand for a long time in the high temperature environment in a charging state. For example, when the lithium rechargeable battery is used in the high temperature environment at 50° C. to 90° C., which is a possible temperature inside the laptop PC, the car, or the like or left to stand for a long time in the charging state, the porous film is gradually deformed or deteriorated by oxidation due to an inner tension or pressure of an electrode/separator element, so that air permeability of the porous film is reduced (i.e. a Gurley value is increased), resulting in reduction in battery life. Accordingly, in recent years, there has been a strong demand for development of a lithium rechargeable battery capable of enduring a use in a high temperature environment of about 90° C. and standing in charging state as well as of maintaining high battery characteristics.    Patent Publication 1: JP-A-09-161814    Patent Publication 2: JP-A-11-329439    Patent Publication 3: JP-A-10-172606    Patent Publication 4: JP-A-09-012756    Patent Publication 5: JP-A-05-310989